Remove the slot_no member of struct thr_local_struct.

enum srv_thread_type: Remove unused thread types.
srv_get_thread_type(): Unused function, remove.

thr_local_get_slot_no(), thr_local_set_slot_no(): Remove.

srv_thread_type_validate(), srv_slot_get_type(): New functions, for debugging.

srv_table_reserve_slot(): Return the srv_slot_t* directly. Do not create
thread-local storage.

srv_suspend_thread(): Get the srv_slot_t* as parameter. Return void;
the caller knows slot->event already.

srv_thread_has_reserved_slot(), srv_release_threads(): Assert
srv_thread_type_validate(type).

srv_init(): Use mem_zalloc() instead of mem_alloc(). Replace
srv_table_get_nth_slot(), because it now asserts that the kernel_mutex
is being held.

srv_master_thread(), srv_purge_thread(): Remember the slot from
srv_table_reserve_slot().

rb:629 approved by Inaam Rana